Aims: Prior research has demonstrated that currently available total ankle implants fail to restore physiologic joint mobility. Most of the modern mobile-bearing designs that feature a flat tibial component and a talar component with anatomic curvature in the sagittal plane function non physiologically with the natural ligament apparatus. The aims of this investigation were a) to elucidate the natural relationship between ligaments and articular surfaces at the intact human ankle joint and b) to develop a new design of total ankle replacement able to replicate this relationship between the retained ligaments and the implanted prosthetic components.

Methods: Motion during passive flexion was analyzed in ten skeleto-ligamentous lower leg preparations including tibia, fibula, talus, calcaneus and intact ligaments. Geometry of ligament fiber arrangement and articular surface shapes was obtained with a 3D digitizer (FARO Technologies, Inc.). A sagittal four-bar linkage model was formulated as formed by the tibia/fibula and talus/ calcaneus rigid segments and by the calcaneofibular and tibiocalcaneal ligaments. To test the ability of possible new prostheses to reproduce the compatible mutual function between the articulating surfaces and the ligaments retained, non-conforming two-component and fully-conforming three-component designs were analyzed. A new total ankle replacement has been designed, prototypes manufactured and implanted in seven skeleto-ligamentous lower leg preparations, and motion was observed. A corresponding new prosthesis has been produced (Finsbury, UK), and implanted in four patients.

Results: The articular surfaces and the ligaments alone prescribed joint motion into a preferred single path of multiaxial rotation (one degree of unresisted freedom). Fibers within the calcaneofibular and tibiocalcaneal ligaments remained most isometric throughout the passive range. The four-bar linkage model well predicted the sagittal plane kinematics observed in corresponding experiments. A ligament-compatible, convex-tibia, fully-congruent, three-component prosthesis design showed the best features: complete congruence over the entire range of flexion together with an acceptable degree of entrapment of the meniscal bearing. Restoration of natural joint kinematics and ligament recruitment was observed in all replaced ankles.

Conclusions: The overall investigation is demonstrating that a profound knowledge of the changing geometry of the joint passive structures throughout the range of passive flexion (mobility) is mandatory for a successful design of joint replacements.

Aims:Understanding total knee replacement mechanics and their influence on patient mobility requires accurate analysis of both operated joint accurate kinematics and full body kinematics and kinetics. The main aim of this study is to perform these two analyses conjointly, as never been reported previously. An innovative graphic-based interface is also pursued aimed at supporting quantitative functional assessment of these patients during the execution of daily living motor tasks in a single synchronized view.

Methods: Three-dimensional fluoroscopic and gait analysis were carried out on eleven patients with PCL-retaining mobile bearing (Interax ISA, Stryker / How-medica / Ostetonics) and on ten posterior stabilized fixed bearing (Optetrak PS, Exactech) knee prostheses. Patients performed three trials of stair ascent twice on the same day: first in the radiology department for fluoroscopy acquisition and later in the Movement Analysis Laboratory, utilizing an identical staircase. Three-dimensional fluoroscopic analysis entails reconstruction of absolute and relative positions and orientations of the two metal components in space by analyzing series of fluoroscopic images of the operated knee and utilizing knowledge of the 3D cad models of these components. Conventional stereophotogrammetry and dynamometry were used to calculate kinematics and kinetics of the trunk, pelvis and of the major joints of the lower limb. An advanced computer-based interface was developed (MULTIMOD, EU-funded project: IST-2000-28377) to show together a) original video of the patient tasks, b) 3D graphical representation of bony segment motion, c) original fluoroscopic images, d) 3D reconstruction of prosthesis component relative motion, and e) graphical transverse plane representation of the contact areas at the base-plate of the replaced knee. All these were registered in space and synchronized in time.

Results: No significant statistical differences on clinical data were found between the two patient populations. Observations at the interface allowed distinct identification of the most critical phases of the task and of the most common compensatory mechanisms utilized by these patients. Statistically significant correlation was found between knee flexion at foot strike and the position of the mid-condylar contact points, and between maximum knee adduction moment and corresponding lateral trunk tilt.

Conclusions: A more complete and powerful assessment of the functional performances of different TKR designs is obtained by combining gait and fluoroscopic in-vivo analyses, which provide correlated and synergic quantitative information.

The aims of Total Knee Arthroplasty (TKA) are to relieve pain and to recreate joint function and stability. Knee joint stability is intimately associated with the concept of joint motion. A stable knee joint is one that maintains an appropriate minimum contact force between the articulating surfaces throughout the functional range of motion of the joint. Thus a TKA is stable when moving through its range of motion it can carry the required functional loads without pain, maintaining contact on non-peripheral located regions and produce joint contact force of normal intensity on the polyethylene insert. Any factors causing an abnormal joint contact force and/or abnormal eccentric position of joint contact force might lead to polyethylene and component loosening. The TKA stability and function are strictly related to the interplay among the implant component alignment, articular surface geometry (flat or congruent polyethylene insert), cruciate-retaining or cruciate-substituting prosthesis design, soft tissue balancing and muscle action. Tibial component loosening continues to be a common mode of failure following TKA. Tibial component fixation is critically dependent on an equilibrium between the mechanical loads and bone resistance to them. Even if it is difficult to find a strict correlation between locomotor lower limb function and knee kinematics, TKR kinematics and position of the point of contact between femur and tibial insert are fundamental biomechanical parameters to understand the reason of extensor mechanism deficit often found in TKA patients and the risk of polyethylene wear. In the present study we will present TKA kinematics and position of the point of contact between femur and tibial insert in fixed and mobile insert focusing in TKA design features. Different knee joint kinematic patterns has been found between fixed and mobile TKA design particularly when congruent artificial joint surface is coupled with mechanical constraint such as the spine-cam mechanism.

Only recently has the mobility of the ankle joint been elucidated. Sliding/rolling of the articular surfaces and slackening/tightening of the ligaments have been explained in terms of a mechanism guided by the isometric rotation of fibres within the calcaneofibular and tibiocalcaneal ligaments. The purpose of this investigation was to design a novel ankle prosthesis able to reproduce this physiological mobility.

A four-bar linkage computer-based model was used to calculate the shapes of talar components compatible with concave, flat and convex tibial components and appropriate fully congruous meniscal bearings. Three-component designs were analysed, and full congruence of the articular surfaces, appropriate entrapment of the meniscal bearing and isometry of the two ligaments were required.

A convex tibial component with 5 cm arc radius gave a 2 mm entrapment together with a 9.8 mm amount of tibial bone cut, while maintaining ligament elongation within 0.03 % of the original length. The physiological patterns of joint motion and ligament tensioning were replicated. The talar component slid backwards while rolling forwards during dorsiflexion. These movements were accommodated by the forward displacement of the meniscal bearing on the tibial surface under the control of the ligaments. The complementary surfaces provide complete congruence over the entire range of flexion, such that a large contact area is achieved in all positions.

To restore the physiological mobility at the ankle joint, not only should the components be designed to be compatible with original ligament pattern of tensioning, but also these should be mounted in the appropriate position. A suitable surgical technique was devised and relevant instrumentation was manufactured. Five below-knee amputated specimens replaced with corresponding prototype components showed good agreement with the model predictions.

Current three-component designs using a flat tibial component and physiological talar shapes cannot be compatible with physiological ligament function.